Plastic parts have the advantage of light weight, corrosion resistance and lower cost. One method for manufacturing some plastic parts is known as rotational molding, or rotomolding. Rotational molding is a process in which a hollow mold is loaded with a pre-measured plastic resin, and the mold is heated as it is slowly rotated bi-axially. The simultaneous heating and rotation causes the melting resin to generally coat the entire inner surface of the mold and fuse. The mold is then cooled and the plastic part is removed from the mold.
Rotational molding results in seamless parts with generally uniform wall thickness and more material in corners to absorb shocks and stresses where they occur most. Rotational molding also offers superb design flexibility and precision. Complex contours, metal inserts, flanges, and molded-in threads can be designed into the walls thus requiring fewer steps to produce the finished product. Since they are typically lighter in weight than metal or fiberglass, rotationally molded finished products are easy to handle and less expensive to ship. In addition, the molds don't need to be designed to withstand the high pressures of injection molding, nor do the molds have an internal core to manufacture. Accordingly, tool costs for the molds are lower relative to other plastic molding techniques, and minor changes can be readily incorporated into existing molds.
Relatively large hollow walled articles, such as tanks for carrying liquids, have been integrally molded utilizing rotational molding processes. Indeed, rotational molding is a cost-effective way to produce such large hollow walled tanks. Although rotationally molded products are recognized for their strength and durability, rotationally molded tanks utilized for liquid transport and storage suffer from a number of problems.
For example, the force exerted on the inner surface of the tank by the liquid can adversely affect the structural integrity of the tank. That is, the tank may bulge outwardly in response to the force and may fail causing the tank to burst. This problem is exacerbated when the tank is large. For example, with a weight of over eight pounds per gallon for water, the overall weight of a load of water in a tank having a storage capacity of as little as two hundred gallons of water can exceed sixteen hundred pounds.
Another problem that arises is that the liquid within the tank acquires momentum as it is transported by a vehicle. As the vehicle accelerates or decelerates, the inertia of the liquid causes it to slosh against the front end or the rear end of the tank. The forces associated with this sloshing further stresses the structural integrity of the tank. More critically, the forces associated with this sloshing can have a destabilizing effect on the vehicle. Such destabilization may cause the driver to lose control of the vehicle, or may cause the vehicle, or a trailer carrying the tank, to overturn.
It is known to place fixed baffles in a tank that extend at right angles to the direction of anticipated movement of liquids contained within the tank. The intent of such baffles is to break the total water volume into smaller volumes thus limiting the distance that the liquid can slosh within the tank. Other prior art systems utilize a floating baffle system of individual spherical objects, each of which floats in the liquid and absorbs the kinetic energy present within the liquid by collision of those objects among themselves.
While both types of baffle systems may be helpful for limiting the movement of the liquid within the tank, they are typically formed from numerous parts that are installed into the tank following manufacturing of the tank itself. The numerous elements and post-manufacturing installation drive up the overall cost and complexity of the tank. In addition, such baffles do not typically strengthen the tank in order to withstand the forces imposed on the tank walls by the liquid.
Accordingly, what is needed is a cost-effectively manufactured hollow walled structure that may be utilized to store and/or transport liquid or solid materials. What is further needed is a tank that can withstand forces imposed on it by the material, as well as provide baffling, so as to limit movement of a liquid within the tank.